- Email: [email protected]
Excited states absorption spectra of porphyrins – Solvent effects
Accepted Manuscript Excited states absorption spectra of porphyrins – solvent effects N.M. Barbosa Neto, D.S. Correa, L. De Boni, G.G. Parra, L. Misoguti, C.R. Mendonça, I.E. Borissevitch, S.C. Zílio, P.J. Gonçalves PII: DOI: Reference:
S0009-2614(13)01239-6 http://dx.doi.org/10.1016/j.cplett.2013.09.066 CPLETT 31630
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
6 July 2013 27 September 2013
Please cite this article as: N.M. Barbosa Neto, D.S. Correa, L. De Boni, G.G. Parra, L. Misoguti, C.R. Mendonça, I.E. Borissevitch, S.C. Zílio, P.J. Gonçalves, Excited states absorption spectra of porphyrins – solvent effects, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.09.066
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Excited states absorption spectra of porphyrins – solvent effects
N. M. Barbosa Neto1, D. S. Correa2, L. De Boni3, G. G. Parra4, L. Misoguti3, C. R. Mendonça3, I. E. Borissevitch4, S. C. Zílio 3, P. J. Gonçalves5* 1
Instituto de Física, Universidade Federal de Uberlândia,Av. João Naves de Ávilla, 2121,
Bloco 1X, 38.400-902 Uberlândia, MG, Brazil. 2
EMBRAPA Instrumentação, Rua XV de Novembro, 1452, CP 741, 13560-970,
São
Carlos, SP, Brazil 3
Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970
São Carlos, SP, Brazil 4
Departamento de Física, FFLCRP, Universidade de São Paulo, Av. Bandeirantes 3900,
14040-901, Ribeirão Preto, SP, Brazil 5
Instituto de Física, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia,
GO, Brazil
1
ABSTRACT
We propose a combination of different techniques to obtain the absorption cross-section spectra of triplet- and singlet-states in porphyrin-like molecules. Two extensions of the Z–scan technique are employed (pulse train Z-scan and white-light Z-scan) together with absorbance spectroscopy and laser flash photolysis. This approach allowed us to investigate features of excited-states absorption of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) that revealed the influence of solvents on the vibronic structuration of the transitions assigned to triplet-states, probably caused by hydrogen bonds established between the porphyrin and solvent molecules.
Keywords: environment effect, excited state absorption, Z-scan, laser flash photolysis, Photodynamic Therapy, PDT. *
Corresponding author: Fax: +55-62-3521-1014, ext 247. E-mail: [email protected]
2
I.
INTRODUCTION
The progress of photonics in the past decades has stimulated the search for materials with high optical nonlinearities aimed at technical applications. Nowadays, a number of studies of photophysical and nonlinear optical (NLO) properties of new promising compounds are performed [1-8] and new techniques for their proper characterization have been developed [912]. Amongst the molecules investigated, those possessing macrocyclic structures like porphyrins, chlorins and phtalocyanines have received considerable attention owing to their potential in applications involving the photodynamic action in biological systems [13-19] and for their favorable NLO properties [20,21]. Particularly, the excited-state absorption (ESA) properties are object of several investigations due to their importance in photonics applications such as optical limiting [22,24], switching [24,25] and others. ESA can be attributed to excited singlet or triplet states absorption, or both simultaneously. Furthermore, the intense absorption in the visible spectral region makes them important molecules for a variety of natural processes, like the energy transport in photosynthesis [26]. The investigation of ESA properties of molecular species is a necessary but quite difficult task to be accomplished, which fostered the development of several techniques, like laser flash photolysis [27-29], picosecond and femtosecond pump and probe [30,31], and Z-scan and their variants [32-34]. Laser flash photolysis (LFP) is a technique employed to investigate transient states created by a nanosecond-pulsed laser [27-29]. These states have absorption characteristics different from those of the ground-state, allowing their investigation through the analysis of the transient absorption spectrum. Such measurement gives information about the kinetic of these states and the absorption difference between the ground and triplet states. However, the transient absorption signal is not capable of determining a number of important information like the intensity of the excited-state transition and the vibronic structure of the
3
spectrum, which are crucial quantities for the complete understanding of the electronic structure of excited-states. Recently, the effects of the environment, and the interaction with gel and liquid vesicles, on the excited-state dynamics of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) were investigated [35,36]. Informations like lifetimes, rate constants, quantum yields and absorption cross-sections were obtained at 532 nm by means of the pulse train Z-scan (PTZ-scan) and spectroscopic techniques [11,35]. However, it is well known that most applications of porphyrins are related to electronic transitions occurring in a broad spectral range, particularly in the Q-band region, which makes the investigation of the excited-state properties of porphyrins of great importance. With this concern, the present work uses an approach that associates LFP and two variants of the Z-scan technique to determine the singlet and triplet excited-state absorption spectrum of TMPyP between 450 and 700 nm. We employed this experimental strategy to investigate the influence of different solvents on the ground-, singlet and triplet excited-states electronic transitions of TMPyP. It is worth to mention that the study of absorption features with the purpose of corr elating them to the molecular structure, and to understand how they respond to external factors, is a research field far from being completely exhausted. We have observed that the vibronic structure of triplet excited-states suffers great influence from the environment, probably due to the presence of hydrogen bonds between the porphyrin and solvent molecules. In contrast, transitions assigned to singlet excited-states (S1→Sn) always present a vibronic structure similar to that observed for transitions coming from the singlet ground-state (S0→S1), corroborating results obtained for other porphyrins [37,38], where no appreciable influence of the environment was observed.
4
II. EXPERIMENTAL SECTION The water soluble cationic meso-tetrakis(4-N-methyl-pyridiniumyl) (TMPyP) porphyrin was purchased from Porphyrin Products Inc and dissolved in Milli-Q quality water. The TMPyP porphyrin concentration was controlled by monitoring its absorption with a Beckman DU 640 spectrophotometer. The linear dependence of the absorbance spectrum on the porphyrin concentration demonstrated that TMPyP does not form aggregates for the concentration range employed [39-41]. Protic methyl (MeOH), ethyl (EtOH) and propyl (PrOH) alcohols, and aprotic acetonitrile were used to investigate the porphyrin excited-state characteristics in polar media. The samples were prepared by adding volumes of TMPyP aqueous stock solution to the organic solvents, yielding a final water content of less than 5%. The TMPyP stock solution concentration was also determined with a spectrophotometer. The ground-state absorption spectra were measured by means of a Cary 17 spectrometer, with the samples placed in a 2 mm-thick quartz cuvette. The singlet excited-state absorption was measured with the white-light continuum (WLC) Z-scan technique [9,34], which uses a broadband pulse instead of the single wavelength employed in the traditional Z-scan technique [32]. The pulse fluence is varied by moving the sample along the focused WLC Gaussian beam (z-direction), reaching the maximum value at the focus. We used the open-aperture configuration to probe signal changes associated only to the nonlinear optical absorption [32]. In this case, all light is focused on the entrance of a portable spectrometer (HR4000 from Ocean Optics). For each sample’s position, the transmittance is normalized to that obtained far from the focal plane, where excited-state contributions to the optical absorption can be neglected. To generate the WLC pulse, 150-fs laser pulses at 775 nm (Ti:sapphire, chirped pulse amplified system) and 1 kHz repetition rate are employed. The pulses are focused (f = 10 cm) into a 3 cm-thick quartz cell containing distilled water. A low-pass optical filter is used to remove the strong pump pulse and the
5
infrared portion of the WLC spectrum. Full details about the WLC Z-scan technique, specially the procedure employed to avoid cumulative contributions, can be found elsewhere [34,38,42]. In the LFP technique the samples are excited by the second harmonic (532 nm) of a Nd:YAG laser (SL400 from Spectron Laser Systems), with 4 ns of pulse duration. The transient spectrum is probed with light from a xenon lamp that illuminates the sample in a direction orthogonal to the pump pulse, which after passing through the sample is spectrally dispersed with a monochromator followed by a photomultiplier tube. The absorption crosssection of the triplet state is measured at 532 nm with the PTZ-scan technique [33,35]. This variant of Z-scan technique allows discriminating contributions from singlet and triplet absorption. Complete details about the PTZ-scan are given elsewhere [33,35]. III.
RESULTS AND DISCUSSIONS The results of our measurements will be explained according to the Jablonski diagram
[33,42] shown in Figure 1. It consists of five energy levels that include the ground-state singlet level (S0) and excited singlet (S1 and Sn) and triplet (T1 and Tm) levels, with possible up and downward (radiative and non-radiative), and intersystem crossing transitions among them. In this diagram, σij is the absorption cross-section for transitions from level i to j and τji is the relaxation time from level j to i. Here we are considering that τ13=τisc is the intersystem crossing time from S1 to T1 and τ30=τT is the T1 state lifetime. In order to obtain the excited-states spectra arising from S1→Sn and T1→Tm transitions, we employed the combination of techniques described above. However, as a preliminary step in our approach, the ground-state absorption cross-section spectrum, σ01(λ), is determined by conventional absorbance experiments and the use of the relation: σ 01 (λ ) =
2.303 A(λ ) N
(1),
6
where A(λ) is the absorbance spectrum, N is the concentration (in molecules per cm3) and ℓ is the cuvette optical path. Next, the S1 → Sn spectrum is obtained with the WLC Z-scan technique. As explained in the experimental section, this technique uses a super-continuum light source to excite porphyrins to the first excited singlet state and the signal transmitted through the sample is acquired by a portable spectrometer as the sample translates through the focal plane of a lens [32,34,41]. The excited singlet state absorption cross-section, σ12, is obtained by analyzing the results shown in Figure 2 with a model that considers just the three energy levels corresponding to the left side of the diagram shown in Figure 1. According to this model, the WLC pulse excites molecules from the ground-state S0 level to a vibronic Franck–Condon state in the S1 band. From there, the molecules can be re-excited to a higher lying singlet-state Sn and then relax back to S1 with a time constant τS n . Considering that the intersystem-crossing and S1 → S0 decay times are much longer than the pulse duration [35], and taking into account stimulated emission during the short pulse, the population dynamics can be described by the following set of the rate equations: dnS 0
(
= −W01 (λ , t ) nS 0 − nS1
dt dnS1 dt
(
)
)
(2)
(
)
= W01 (λ , t ) nS 0 − nS1 − W12 (λ , t ) nS1 − nS n +
dn S n dt
(
)
= W12 (λ , t ) n S1 − n S n −
nS n
τS
n Sn
τS
(3)
n
(4)
n
where n S0 , n S1 and n Sn are the population fractions in the ground (S0), first (S1) and higher (Sn) excited singlet states, respectively.
W01(λ , t ) =
σ 01 (λ )I (λ , t ) ω
and
W12 (λ , t ) =
σ 12 (λ )I (λ , t ) ω
are the S0 → S1
and S1 → Sn transition rates, with σ 01 (λ ) and σ 12 (λ ) being the absorption cross-sections
7
corresponding to these transitions. The time dependence introduced in the irradiance, I(λ,t), takes into account the previously determined WLC chirp. These equations are numerically solved using the irradiance extracted from the WLC energy at a given wavelength, which was estimated using a procedure previously described [34]. The calculations were carried out using
σ01(λ) extracted from linear absorbance spectra. The higher excited-state lifetime, τS n , was assumed to be one order of magnitude faster (picosecond scale), in accordance with values found in previous studies [31,35]. It is important to note that the results obtained with this fitting procedure are nearly independent of τS n because the population of the Sn state is always very small because of the low fluence of our excitation source. Considering the population redistribution caused by the strong WLC pulse incident on the sample, the time evolution of the nonlinear absorption can be written as:
α (λ , t ) = N [n0 (t )σ 01 (λ ) + n1 (t )σ 12 (λ )]
(5)
Since the detection system in the experiment measures the pulse fluence, the transmittance is calculated by integrating the propagation equation, dI/dz= -α(t)I(t), over the sample thickness and the full pulse duration, which takes into account accumulative effects. From the absorption spectra measured with the WLC Z-scan and using the cross-section σ01(λ), the
σ12(λ) spectra for TMPyP is obtained by fitting of the normalized transmittance acquired at each wavelength. The results obtained will be presented later, together with the spectra of other excited-sates cross-sections. The triplet-triplet absorption measured with the nanosecond laser flash photolysis at 0.5 µs after excitation is depicted in Figure 3. The pump pulse creates triplet states and induces temporal changes in the molecular absorption behavior. Assuming that the molecules that are pumped out from the S0 ground-state can either return to S0 or populate the lowest triplet state T1, via intersystem crossing process, the probe beam will analyze the redistribution of the
8
population between these states. In this way, the transient absorption spectra, ∆A(λ), can be written as [29]: ∆A(λ ) = [ε 34 (λ ) − ε 01 (λ )]CT
(6)
where ε 01 (λ ) and ε 34 (λ ) are the ground and triplet states molar absorption coefficients, respectively, and CT is triplet molar concentration. The relationship between the molar absorption coefficient and absorption cross-section can be obtained from the Beer law expressed as A(λ)= ε(λ)Cℓ, where C is the molar concentration (in mol.L-1). Since the concentration (in molecules/cm3) is given by N = N A3 C , where NA is Avogadro number, Eq. (1) 10
becomes: σ ij (λ ) =
2303 ε ij (λ ) NA
(7)
where the molar absorption coefficient has M-1cm-1 units while the absorption cross-section is in units of cm2. In this way, Eq. (6) can be rewritten by: ∆A(λ ) =
1 [σ 34 (λ ) − σ 01(λ )]NT 2.303
(8)
Since ∆A(λ) was measured with LFP technique, we could, in principle, determine σ34(λ). However, the triplet population (NT) is unknown, which prevents the determination of σ34(λ) from Eq. (8). Many approaches have been introduced to circumvent this problem [29,43,44]. Among them, the saturation method proposed by Kalyanasundaram and Neumann-Spallart for TMPyP porphyrin [43,44] arises as a possible solution. Basically, this method consists in measuring the transient absorbance for different laser intensities up to saturation, which corresponds to the complete transference of molecules to the triplet state. However, in order to avoid the photolysis of the sample, the spectrum is generally obtained in the infrared region, which is a serious limitation because most applications of porphyrins occur in the visible region of the spectrum, particularly in the Q-band region.
9
In our approach, we use PTZ-scan data to determine the absorption cross-section values (σ01 and σ34) at 532 nm for TMPyP in all solvents [35]. These values are then used to calibrate the LFP results by finding NT through Eq. (8). Since the excitation wavelength used in laser flash photolysis experiments is also at 532 nm, we do not measure ∆A(λ) at this wavelength in order to avoid problems with spurious light scattered from the excitation laser beam. Instead, ∆A(λ) for 532 nm is obtained from an extrapolation from their values at 525 and 540 nm. With the value of NT determined from PTZ-scan data, we can use Eq. (8) again in order to obtain the σ34(λ) spectra from the LFP transient absorption spectrum, as depicted in Figure 4 (green full squares). Concerning to the magnitude and spectral profile of the cross-sections, Figure 2 shows that the normalized transmittance is lower than 1 for all solvents investigated which points out to the fact that the molecule presents the absorption cross-section, σ12(λ), higher than that of the ground-state, σ01(λ), in the visible spectral region. The only exception is observed for wavelengths near 518 nm, where the normalized absorbance close to 1 indicates that the absorption cross-sections of both S0→S1 and S1→Sn transitions have similar values. Despite the characteristics of the solvents, the excited singlet state absorption spectra always present a profile quite similar to that assigned to the ground-state absorption. The only difference is the small enhancement observed in the magnitude of the transitions, as shown in Figure 4. Such enhancement of σ12 relative to σ01 is responsible for the increase in the absorption observed in the normalized transmittance spectra. We have studied the singlet excited-state spectra of several organic molecules during the last few years. Our results for indocyanines, phthalocyanines and bisphthalocyanines show that the S0→S1 and S1→Sn cross-sections spectra profiles obtained are completely different [45-47]. On the other hand, we found that for porphyrins and their derivates, like cytochrome and chlorophyll a, the absorption spectra profile are very similar [37,38,42,48,49]. This type of
10
behavior in porphyrin also was previously reported by others authors in others porphyrins [50,51]. In this way, the vibronic characteristic of the Q-band of porphyrins, for both S0→S1 and S1→Sn, are in fact nearly identical with respect to their structuration. These data and the small fluorescence Stokes shift observed previously [35] can be explained by considering that porphyrins are, in general, rigid structures and that the TMPyP vibronic structure is preserved upon excitation. In this way, the molecular structure is the same for all singlet-states participating in the absorption process. Previous work reported in the literature corroborates the suggestion of a strong vibronic coupling between excited-states and ground-state in porphyrins. For instance, Kumble et. al. [52] observed an overall expansion of the macrocycle upon photoexcitation, but the ground-state normal mode structure was retained in the S1 excited state. The vibrational frequencies of both saturated and unsaturated substituents are found to be minimally perturbed in the S1 excited-states. Improta et. al. [53] also observed that the minimum energy geometry of Qx is similar to ground electronic state, owing to the rigid structure of the free-base porphyrin core. Nguyen et al. [54] observed that the lowest triplet excited-state structure of free-base porphyrin and its octahalogenated derivates retain the D2h symmetry. Besides, Sato et. al. [55] suggested the presence of S1-S2 vibronic coupling and the existence of a hyperconjugation between the macrocycle and ethyl groups in the S0, S1 and T1 states. We also note that, similar to S0 → S1 transitions, almost no difference is observed in the excited singlet state absorption spectra of TMPyP for different solvents, strongly suggesting that no appreciable difference in the σ12/σ01 ratio is caused by the changes in the polarity of the medium. Such behavior is better observed in Figure 5(a). Differently from the absorption assigned to singlet excited state, Figure 4 shows that the triplet-triplet absorption (green solid squares) does not have the same vibronic structure present in singlet transitions for most solvents employed. We can see that T1→Tm transitions present
11
vibronic structures at the Q-band region that are completely different from those of singlet states when water, methyl, ethyl, and propyl alcohol are used as solvents. Furthermore, the transition intensities are enhanced in the blue region of the spectrum. By analyzing the solvents properties, we observe in Table 1 that water present a dipole moment higher than the alcohols, which, in principle, points out to a possible influence of the solvent’s dipole moment on the triplet-triplet transition. However, it is also noted in Table 1 that the alcohols used as solvent (ethanol, methanol and propanol), present similar dipole moments and dissimilar Q-band vibronic structuration (see Figures 4(b), 4(c) and 4(d)). Aiming at a deeper understanding of the influence of the solvent on the vibronic structure of the triplet-triplet absorption at the Q-band region, we dissolved the porphyrin in acetonitrile, which is a polar aprotic solvent that presents dipole moment higher than water. In this case, the vibronic structure of the triplet-triplet absorption is more preserved around the Q-band region, when compared to singlet states transitions, than when water is used as solvent. Such result supports that the vibronic structure of triplet states could mainly be affected not only by the solvent dipole moment, but also can be attributed to hydrogen bonds. According to Gensch et al. [56], the nitrogen lone electron pairs in the porphyrin macrocycle are strongly hydrogen-bounded with the water molecules, in the solvation sphere of the porphyrin. This result is in agreement with a larger electron density at the nitrogen atoms of the porphyrin, as found through studies using resonance Raman studies and semiempirical calculations [57,58]. We have recently shown that the photophysical characteristics of TMPyP porphyrin in organic solvents differ from those in water [35], where the formation of hydrogen bonds between TMPyP nitrogen atoms and water hydrogen can explain results such as spectral changes, blue shift and can also explain the higher T1 →Tm absorption cross-section, mainly in Q bands observed in the present work.
12
Previous studies of structural volume changes upon photo-excitation of porphyrins have suggested their contraction upon population of the triplet state [56,59]. It was proposed that this contraction occurs because of the redistribution of the electronic density in the macrocyclic core, which is not observed when the molecule is in the ground-state. Such redistribution could lead to changes in the strength of the hydrogen bonding, between the water molecules in the hydratation shell and the nitrogen atoms in the of porphyrin macrocycle, leading to the molecular contraction of the aqueous sphere around the macrocycle in the triplet state [59]. Finally, Figure 5 presents the ratio spectra σ12/σ01 (a) and σ34/σ01 (b) for all solvents investigated. Such ratios are critical to evaluate if the material investigated has potential to be employed as a good optical limiter [25]. We can see in both spectra that TMPyP is a reverse saturable absorber, presenting higher values of σ12/σ01 for two ranges: from 450 to 500 nm and from 600 to 700 nm. Moreover, above 600 nm, the ground-state cross-section values are very small, implying in reverse saturable absorbers with high linear transmittance. The σ34/σ01 ratio spectrum presents high values between 450 and 500 nm, with an average value close to 15, and reaching the highest value (nearly 26) when the porphyrin is dissolved in methanol. This behavior indicates its potential to optical limiting applications for this spectral region in the nanosecond scale.
IV.
CONCLUSIONS In summary, a new approach to obtain the singlet-singlet and the triplet-triplet excited
states absorption of porphyrins has been presented. In such approach, we employed an association of laser flash photolysis with two extensions of the Z-scan technique to measure both the singlet-singlet and the triplet-triplet excited-state absorption spectra. Employing this method we were able to investigate the influence of different solvents on the ground as well as excited electronic transitions of TMPyP. The results show that S1 → S n spectra have the same
13
vibronic structure as S0 → S1, with similar spectral shape and magnitude, in all solvents employed. However, the T1 → Tm transition shows a quite different vibronic structuration when compared to transitions assigned to singlet states. Finally, our results show that the modification of the T1 → Tm vibronic structure is affected not only by the solvent dipole moment, but also probably by hydrogen bonds established between the porphyrin and solvent molecules.
Acknowledgments - We acknowledge the support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) and National Institute for Photonics (INCT/INFo). The authors are also in debt with the referees for valuable suggestions.
References [1] D. Koszelewski, A. Nowak-Król, M. Drobizhev, C. J. Wilson, J. E. Haley, T. M. Cooper, J. Romiszewski, E. Górecka, H. L. Anderson, A. Rebane, D. T. Gryko, J. Mater. Chem. C 1 (2013) 2044. [2] E. Güzel, A. Atsay, S. Nalbantoglu, N. Saki, A. L. Dogan, A. Gül, M. B. Koçak, Dyes and Pigments 97 (2013) 238. [3] A. Marcelli, I. J. Badovinac, N. Orlic, P. R. Salvi, Cristina Gellini, Photochem. Photobiol. Sci. 12 (2013) 348. [4] Z. –B. Liu, Z. Guo, X. –L. Zhang, J.-Y. Zheng, J. -G. Tian, Carbon 51 (2013) 419. [5] H. Wang, D. Qi, Z. Xie, W. Cao, K. Wang, H. Shang, J. Jiang, Chem. Commun. 49 (2013) 889.
14
[6] Y. Dwivedi, L. de Boni, P.J. Gonçalves, L.M. Mairink, R. Menegatti, T.L. Fonseca, S.C. Zilio, Spectrochim. Acta A 105 (2013) 483. [7] C. J. P. Monteiro, J. Pina, M. M. Pereira, L. G. Arnaut, Photochem. Photobiol. Sci. 11 (2012) 1223. [8] R. N. Sampaio, W. R. Gomes, D. M. S. Araujo, A. E. H. Machado, R. A. Silva, A. Marletta, I. E. Borissevitch, A. S. Ito, L. R. Dinelli, A. A. Batista, S. C. Zílio, P. J. Gonçalves, N. M. Barbosa Neto, J. Phys. Chem. A 116 (2012) 18. [9] M. Balu, J. Hales, D. J. Hagan, E.W. Van Stryland, Opt. Express 12 (2004) 3820. [10] K. Clays, A. Persoons, Rev. Sci. Instrum. 63 (1992) 3285. [11] L. De Boni, P. L. Franzen, P. J. Gonçalves, I. E. Borissevitch, L. Misoguti, C. R. Mendonça, S. C. Zilio, Opt. Express 19 (2011) 10813. [12] M. Pineiro, A. L. Carvalho, M. M. Pereira, A. M. R. Gonsalves, L. G. Arnaut, S. J. Formosinho, Chem. Eur. J. 4 (1998) 2299. [13] S. M. Abdelghany, D. Schmid, J. Deacon, J. Jaworski, F. Fay, K. M. McLaughlin, J. A. Gormley, J.F. Burrows, D. B. Longley, R. F. Donnelly, C. J. Scott, Biomacromolecules 14 (2013) 302. [14] A. Eichner, F. P. Gonzales, A. Felgenträger, J. Regensburger, T. Holzmann, W. Schneider-Brachert, W. Bäumler, T. Maisch, Photochem. Photobiol. Sci. 12 (2013) 135. [15] J. T. -W. Wang, K. Berg, A. Høgset, S. G. Bown, A. J. MacRobert, Photochem. Photobiol. Sci. 12 (2013) 519. [16] L. M. Almeida, F. F. Zanoelo, K. P. Castro, I. E. Borissevitch, C. M. A. Soares, P. J. Gonçalves, Photochem. Photobiol. 88 (2012) 992. [17] G. B. Rodrigues, F. L. Primo, A. C. Tedesco, G. U. L. Braga, Photochem. Photobiol. 88 (2012) 440. [18] M. Ethirajan, Y. Chen, P. Joshi, R. K. Pandey, Chem. Soc. Rev. 40 (2011) 340.
15
[19] J. M. Dabrowski, L. G. Arnaut, M. M. Pereira, C. J. P. Monteiro, K. Urbańska, S. Simões, G. Stochel, ChemMedChem. 5 (2010) 1770. [20] L. De Boni, D. S. Correa, D. L. Silva, P. J. Gonçalves, S. C. Zilio, G. G. Parra, I. E. Borissevitch, S. Canuto, C. R. Mendonca, J. Chem. Phys. 134 (2011) 014509. [21] F. Gao, X. Wang, S. Wang, M. Liu, X. Liu, X. Ye, H. Li, Tetrahedron 69 (2013) 2720. [22] S. M. O’Flaherty, S. V. Hold, M. J. Cook, T. Torres, Y. Chen, M. Hanack, W. J. Blau, Adv. Mat. 15 (2003) 19. [23] N. M. Barbosa Neto, S. L. Oliveira, L. Misoguti, C. R. Mendonca, P. J. Goncalves, I. E. Borissevitch, L. R. Dinelli, L. L. Romualdo, A. A. Batista, S. C. Zilio, J. Appl. Phys. 99 (2006) 123103. [24] G. de la Torre, P. Vaquez, F. Agullo-Lopez, T. Torres, Chem. Rev. 104 (2004) 3723. [25] C. Loppacher, M. Guggisberg, O. Pfeiffer, E. Meyer, M. Bammerlin, R. Lüthi, R. Schlittler, J. K. Gimzewski, H. Tang, C. Joachim, Phys. Rev. Lett. 90 (2003) 066107-1. [26] D. Voet and J. G. Voet, Biochemistry, John Wiley & Sons Inc. 2o ed., New York, 1995. [27] B. Amand and R. Bensasson, Chem. Phys. Lett. 34 (1975) 44. [28] R. H. Compton, K. T. V. Grattan, T. Morrow, J. Photochem. 14 (1980) 61. [29] I. Carmichael, G. L. Hug, J. Phys. Chem. Ref. Data 15 (1986) 1. [30] H. Gratz, A. Penzkofer, J. Photochem. Photobiol. A 127 (1999) 21. [31] M. Enescu, K. Steenkeste, F. Tfibel, M. P. Fontaine-Aupart, Phys. Chem. Chem. Phys. 4 (2002) 6092. [32] M. Sheik-Bahae, A. A. Said, T. –H. Wei, D. J. Hagan, E. W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760. [33] L. Misoguti, C. R. Mendonça, S. C. Zilio, Appl. Phys. Lett. 74 (1999) 1531. [34] L. De Boni, A. A. Andrade, L. Misoguti, C. R. Mendonça, S. C. Zílio, Opt. Express 12 (2004) 3921.
16
[35] P. J. Gonçalves, P. L. Franzen, D. S. Correa, L. M. Almeida, M. Takara, A. S. Ito, S. C. Zílio, I. E. Borissevitch, Spectrochim. Act. A 79 (2011) 1532. [36] D. S. Neto, M. Tabak, J. Colloid Interf. Sci. 381 (2012) 73. [37] R. V. Maximiano, E. Piovesan, S. C. Zílio, A. E. H. Machado, R. de Paula, J. A. S. Cavaleiro, I. E. Borissevitch, A. S. Ito, P. J. Gonçalves, N. M. Barbosa Neto, J. Photochem. Photobiol. A 214 (2010) 115. [38] P. J. Gonçalves, I. E. Borissevitch, S. C. Zilio, Chem. Phys. Lett. 469 (2009) 270. [39] J. Mosinger, M. Janoskova, K. Lang, P. Kubat, J. Photochem. Photobiol. A 181 (2006) 283. [40] P. Kubat, K. Lang, K. Prochazkova, P. Anzenbacher Jr, Langmuir 19 (2003) 422. [41] P. J. Gonçalves, N. M. Barbosa Neto, G. G. Parra, L. de Boni, L. P. F. Aggarwal, J. P. Siqueira, L. Misoguti, I. E. Borissevitch, S. C. Zílio, Opt. Mater. 34 (2012) 741. [42] P. J. Gonçalves, D. S. Corrêa, P. L. Franzen, L. De Boni, C. R. Mendonça, I. E. Borissevitch, S. C. Zílio, Spectrochim. Acta A 12 (2013) 309. [43] K. Kalyanasundaram, Inorg. Chem. 23 (1984) 2453. [44] K. Kalyanasundaram, M. Neumann-Spallart, J. Phys. Chem. 86 (1982) 5163. [45] L. De Boni, E. Piovesan, L. Gaffo, C. R. Mendonça, J. Phys. Chem. A 112 (2008) 6803. [46] M. G. Vivas, E. G. R. Fernandes, M. L. Rodríguez-Méndez, C. R. Mendonca, Chem. Phys. Lett., 531 (2012) 173. [47] L. De Boni, C. R. Mendonca, J. Phys. Org. Chem. 24 (2011) 630. [48] L. De Boni, A.A. Andrade, L. Misoguti, S.C. Zílio, C. R. Mendonça, Opt. Mater. 32 (2010) 526. [49] L. De Boni, D. S. Correa, F. J. Pavinatto, D. S. dos Santos, Jr., C. R. Mendonça, J. Chem. Phys. 126 (2007) 165102. [50] H. Gratz, A. Penzkofer, Chem. Phys. 254 (2000) 363.
17
[51] Z. –B. Liu, Y. Zhu, Y. –Z. Zhu, J. -G. Tian, J. –Y. Zheng, J. Phys. Chem. B 111 (2007) 14136. [52] R. Kumble, G. R. Loppnow, S. Hu, A. Mukherjee, M. A. Thompson, T. G. Spiro, J. Phys. Chem. 99 (1995) 5809. [53] R. Improta, C. Ferrante, R. Bozio, V. Barone, Phys. Chem. Chem. Phys. 11 (2009) 4664. [54] K. A. Nguyen, P. N. Day, R. Pachter, J. Phys. Chem. A 104 (2000) 4748. [55] S. Sato, K. Aoyagi, T. Haya, T. Kitagawa, J. Phys. Chem. 99 (1995) 7766. [56] T. Gensch, C. Viappiani, S.E. Braslavsky, J. Am. Chem. Soc. 121 (1999) 10573. [57] S. E. J. Bell, C. B. Aakeroy, A. H. R. Al-Obaidi, J. N. M. Hegarty, J. J. McGarvey, C. R. Lefley, J. N. Moore, R. E. Hester, J. Chem. Soc., Faraday Trans. 91 (1995) 411. [58] S. E. J. Bell, A. H. R. Al-Obaidi, J. N. M. Hegarty, R. E. Hester, J. J. McGarvey, J. Phys. Chem. 97 (1993) 11599 [59] M. M. Kruk, S. E. Braslasky, Photochem. Photobiol. Sci. 11 (2012) 972.
18
Figure Captions
Figure 1. Five-energy-level diagram.
Figure 2. Normalized transmittance spectra for TMPyP in the presence of solvents with different polarities.
Figure 3. Absorption spectra obtained with LFP at 0.5 µs after excitation for TMPyP in the presence of solvents with different polarities.
Figure 4. Absorption cross-section spectra of ground (σ01, inverted full triangles), first excited singlet (σ12, open red circles) and first excited triplet states (σ34, full green squares) obtained for TMPyP in different polar protic solvents: (a) water, (b) ethyl, (c) methyl, (d) propyl alcohol and (e) acetonitrile.
Figure 5. (a) σ12/σ01 and (b) σ34/σ01 spectra for all solvents investigated.
19
Figure 1. N. M. Barbosa Neto et. al.
Normalized Transmittance
1.00
0.95 H2 0 0.90
Acetonitrile MeOH EtOH PrOH
0.85
0.80 450
500
550
600
Wavelength (nm) Figure 2. N. M. Barbosa Neto et. al.
650
700
A
0.16
H2 O
0.12
Acetonitrile MeOH EtOH PrOH
0.08
0.04
0.00 400
500
600
Wavelength (nm) Figure 3. N. M. Barbosa Neto et. al.
700
2
cm )
10
12()
6
34()
4 2 0 500
550
600
650
700
Wavelength (nm)
5 0
40
8
01()
2 -17
ij - cross-section (10
35
cm )
(b)
30 25 20
6
12() 34()
4
2
0 500
15
550
600
650
700
Wavelength (nm)
10 5 0 15
8
01()
ij - cross-section(10
-17
2
cm )
(c) 10
6
12() 34()
4
2
0 500
5
0 25
-17
2
cm )
(d)
ij - cross-section (10
20 15 10
550
600
Wavelength (nm)
650
8
700
01()
6
12()
4
34()
2
0 500
550
600
650
700
Wavelength (nm)
5 0 20
8
01( )
2
cm )
2
(e)
-17
-17
ij - cross-section (10
01()
8
-17
15
cm )
-17
ij - cross-section (10
10
ij - cross-section (10
20
6
ij - cross-section(10
2
cm ) ij - cross-section (10-17 cm2)
-17
(a)
2
cm ) - cross-section (10-17 cm2) ij - cross-section (10 ij
25
15
10
12( ) 34( )
4
2
0 500
550
600
Wavelength (nm)
650
700
5
0 450
500
550
600
650
Wavelength (nm)
Figure 4. N. M. Barbosa Neto et. al.
700
21/01
2.5
(a)
H2O MeOH EtOH PrOH Acet.
2.0
1.5
1.0
30
(b)
H2O MeOH EtOH PrOH Acet.
25
34/01
20 15 10 5 0 450
500
550
600
650
Wavelength (nm) Figure 5. N. M. Barbosa Neto et. al.
700
Table 1.
Solvent Dipole moment (D) Type Water 1.85 Polar protic Ethanol 1.69 Polar protic Methanol 1.70 Polar protic Propanol 1.68 Polar protic Acetonitrile 3.92 Polar aprotic
20
Highlights We have obtained the excited states spectra of triplet and singlet states. It was employed UV/Vis absorption, flash photolysis and Z–scan techniques. S1 → Sn absorption spectra were obtained by white-light Z-scan technique. T1 → Tm absorption was obtained by flash photolysis and pulse train Z-scan. The vibronic structuration observed is probably caused by hydrogen bonds.
21
22
S0009-2614(13)01239-6 http://dx.doi.org/10.1016/j.cplett.2013.09.066 CPLETT 31630
To appear in:
Chemical Physics Letters
Received Date: Accepted Date:
6 July 2013 27 September 2013
Please cite this article as: N.M. Barbosa Neto, D.S. Correa, L. De Boni, G.G. Parra, L. Misoguti, C.R. Mendonça, I.E. Borissevitch, S.C. Zílio, P.J. Gonçalves, Excited states absorption spectra of porphyrins – solvent effects, Chemical Physics Letters (2013), doi: http://dx.doi.org/10.1016/j.cplett.2013.09.066
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Excited states absorption spectra of porphyrins – solvent effects
N. M. Barbosa Neto1, D. S. Correa2, L. De Boni3, G. G. Parra4, L. Misoguti3, C. R. Mendonça3, I. E. Borissevitch4, S. C. Zílio 3, P. J. Gonçalves5* 1
Instituto de Física, Universidade Federal de Uberlândia,Av. João Naves de Ávilla, 2121,
Bloco 1X, 38.400-902 Uberlândia, MG, Brazil. 2
EMBRAPA Instrumentação, Rua XV de Novembro, 1452, CP 741, 13560-970,
São
Carlos, SP, Brazil 3
Instituto de Física de São Carlos, Universidade de São Paulo, Caixa Postal 369, 13560-970
São Carlos, SP, Brazil 4
Departamento de Física, FFLCRP, Universidade de São Paulo, Av. Bandeirantes 3900,
14040-901, Ribeirão Preto, SP, Brazil 5
Instituto de Física, Universidade Federal de Goiás, Caixa Postal 131, 74001-970 Goiânia,
GO, Brazil
1
ABSTRACT
We propose a combination of different techniques to obtain the absorption cross-section spectra of triplet- and singlet-states in porphyrin-like molecules. Two extensions of the Z–scan technique are employed (pulse train Z-scan and white-light Z-scan) together with absorbance spectroscopy and laser flash photolysis. This approach allowed us to investigate features of excited-states absorption of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) that revealed the influence of solvents on the vibronic structuration of the transitions assigned to triplet-states, probably caused by hydrogen bonds established between the porphyrin and solvent molecules.
Keywords: environment effect, excited state absorption, Z-scan, laser flash photolysis, Photodynamic Therapy, PDT. *
Corresponding author: Fax: +55-62-3521-1014, ext 247. E-mail: [email protected]
2
I.
INTRODUCTION
The progress of photonics in the past decades has stimulated the search for materials with high optical nonlinearities aimed at technical applications. Nowadays, a number of studies of photophysical and nonlinear optical (NLO) properties of new promising compounds are performed [1-8] and new techniques for their proper characterization have been developed [912]. Amongst the molecules investigated, those possessing macrocyclic structures like porphyrins, chlorins and phtalocyanines have received considerable attention owing to their potential in applications involving the photodynamic action in biological systems [13-19] and for their favorable NLO properties [20,21]. Particularly, the excited-state absorption (ESA) properties are object of several investigations due to their importance in photonics applications such as optical limiting [22,24], switching [24,25] and others. ESA can be attributed to excited singlet or triplet states absorption, or both simultaneously. Furthermore, the intense absorption in the visible spectral region makes them important molecules for a variety of natural processes, like the energy transport in photosynthesis [26]. The investigation of ESA properties of molecular species is a necessary but quite difficult task to be accomplished, which fostered the development of several techniques, like laser flash photolysis [27-29], picosecond and femtosecond pump and probe [30,31], and Z-scan and their variants [32-34]. Laser flash photolysis (LFP) is a technique employed to investigate transient states created by a nanosecond-pulsed laser [27-29]. These states have absorption characteristics different from those of the ground-state, allowing their investigation through the analysis of the transient absorption spectrum. Such measurement gives information about the kinetic of these states and the absorption difference between the ground and triplet states. However, the transient absorption signal is not capable of determining a number of important information like the intensity of the excited-state transition and the vibronic structure of the
3
spectrum, which are crucial quantities for the complete understanding of the electronic structure of excited-states. Recently, the effects of the environment, and the interaction with gel and liquid vesicles, on the excited-state dynamics of meso-tetrakis (methylpyridiniumyl) porphyrin (TMPyP) were investigated [35,36]. Informations like lifetimes, rate constants, quantum yields and absorption cross-sections were obtained at 532 nm by means of the pulse train Z-scan (PTZ-scan) and spectroscopic techniques [11,35]. However, it is well known that most applications of porphyrins are related to electronic transitions occurring in a broad spectral range, particularly in the Q-band region, which makes the investigation of the excited-state properties of porphyrins of great importance. With this concern, the present work uses an approach that associates LFP and two variants of the Z-scan technique to determine the singlet and triplet excited-state absorption spectrum of TMPyP between 450 and 700 nm. We employed this experimental strategy to investigate the influence of different solvents on the ground-, singlet and triplet excited-states electronic transitions of TMPyP. It is worth to mention that the study of absorption features with the purpose of corr elating them to the molecular structure, and to understand how they respond to external factors, is a research field far from being completely exhausted. We have observed that the vibronic structure of triplet excited-states suffers great influence from the environment, probably due to the presence of hydrogen bonds between the porphyrin and solvent molecules. In contrast, transitions assigned to singlet excited-states (S1→Sn) always present a vibronic structure similar to that observed for transitions coming from the singlet ground-state (S0→S1), corroborating results obtained for other porphyrins [37,38], where no appreciable influence of the environment was observed.
4
II. EXPERIMENTAL SECTION The water soluble cationic meso-tetrakis(4-N-methyl-pyridiniumyl) (TMPyP) porphyrin was purchased from Porphyrin Products Inc and dissolved in Milli-Q quality water. The TMPyP porphyrin concentration was controlled by monitoring its absorption with a Beckman DU 640 spectrophotometer. The linear dependence of the absorbance spectrum on the porphyrin concentration demonstrated that TMPyP does not form aggregates for the concentration range employed [39-41]. Protic methyl (MeOH), ethyl (EtOH) and propyl (PrOH) alcohols, and aprotic acetonitrile were used to investigate the porphyrin excited-state characteristics in polar media. The samples were prepared by adding volumes of TMPyP aqueous stock solution to the organic solvents, yielding a final water content of less than 5%. The TMPyP stock solution concentration was also determined with a spectrophotometer. The ground-state absorption spectra were measured by means of a Cary 17 spectrometer, with the samples placed in a 2 mm-thick quartz cuvette. The singlet excited-state absorption was measured with the white-light continuum (WLC) Z-scan technique [9,34], which uses a broadband pulse instead of the single wavelength employed in the traditional Z-scan technique [32]. The pulse fluence is varied by moving the sample along the focused WLC Gaussian beam (z-direction), reaching the maximum value at the focus. We used the open-aperture configuration to probe signal changes associated only to the nonlinear optical absorption [32]. In this case, all light is focused on the entrance of a portable spectrometer (HR4000 from Ocean Optics). For each sample’s position, the transmittance is normalized to that obtained far from the focal plane, where excited-state contributions to the optical absorption can be neglected. To generate the WLC pulse, 150-fs laser pulses at 775 nm (Ti:sapphire, chirped pulse amplified system) and 1 kHz repetition rate are employed. The pulses are focused (f = 10 cm) into a 3 cm-thick quartz cell containing distilled water. A low-pass optical filter is used to remove the strong pump pulse and the
5
infrared portion of the WLC spectrum. Full details about the WLC Z-scan technique, specially the procedure employed to avoid cumulative contributions, can be found elsewhere [34,38,42]. In the LFP technique the samples are excited by the second harmonic (532 nm) of a Nd:YAG laser (SL400 from Spectron Laser Systems), with 4 ns of pulse duration. The transient spectrum is probed with light from a xenon lamp that illuminates the sample in a direction orthogonal to the pump pulse, which after passing through the sample is spectrally dispersed with a monochromator followed by a photomultiplier tube. The absorption crosssection of the triplet state is measured at 532 nm with the PTZ-scan technique [33,35]. This variant of Z-scan technique allows discriminating contributions from singlet and triplet absorption. Complete details about the PTZ-scan are given elsewhere [33,35]. III.
RESULTS AND DISCUSSIONS The results of our measurements will be explained according to the Jablonski diagram
[33,42] shown in Figure 1. It consists of five energy levels that include the ground-state singlet level (S0) and excited singlet (S1 and Sn) and triplet (T1 and Tm) levels, with possible up and downward (radiative and non-radiative), and intersystem crossing transitions among them. In this diagram, σij is the absorption cross-section for transitions from level i to j and τji is the relaxation time from level j to i. Here we are considering that τ13=τisc is the intersystem crossing time from S1 to T1 and τ30=τT is the T1 state lifetime. In order to obtain the excited-states spectra arising from S1→Sn and T1→Tm transitions, we employed the combination of techniques described above. However, as a preliminary step in our approach, the ground-state absorption cross-section spectrum, σ01(λ), is determined by conventional absorbance experiments and the use of the relation: σ 01 (λ ) =
2.303 A(λ ) N
(1),
6
where A(λ) is the absorbance spectrum, N is the concentration (in molecules per cm3) and ℓ is the cuvette optical path. Next, the S1 → Sn spectrum is obtained with the WLC Z-scan technique. As explained in the experimental section, this technique uses a super-continuum light source to excite porphyrins to the first excited singlet state and the signal transmitted through the sample is acquired by a portable spectrometer as the sample translates through the focal plane of a lens [32,34,41]. The excited singlet state absorption cross-section, σ12, is obtained by analyzing the results shown in Figure 2 with a model that considers just the three energy levels corresponding to the left side of the diagram shown in Figure 1. According to this model, the WLC pulse excites molecules from the ground-state S0 level to a vibronic Franck–Condon state in the S1 band. From there, the molecules can be re-excited to a higher lying singlet-state Sn and then relax back to S1 with a time constant τS n . Considering that the intersystem-crossing and S1 → S0 decay times are much longer than the pulse duration [35], and taking into account stimulated emission during the short pulse, the population dynamics can be described by the following set of the rate equations: dnS 0
(
= −W01 (λ , t ) nS 0 − nS1
dt dnS1 dt
(
)
)
(2)
(
)
= W01 (λ , t ) nS 0 − nS1 − W12 (λ , t ) nS1 − nS n +
dn S n dt
(
)
= W12 (λ , t ) n S1 − n S n −
nS n
τS
n Sn
τS
(3)
n
(4)
n
where n S0 , n S1 and n Sn are the population fractions in the ground (S0), first (S1) and higher (Sn) excited singlet states, respectively.
W01(λ , t ) =
σ 01 (λ )I (λ , t ) ω
and
W12 (λ , t ) =
σ 12 (λ )I (λ , t ) ω
are the S0 → S1
and S1 → Sn transition rates, with σ 01 (λ ) and σ 12 (λ ) being the absorption cross-sections
7
corresponding to these transitions. The time dependence introduced in the irradiance, I(λ,t), takes into account the previously determined WLC chirp. These equations are numerically solved using the irradiance extracted from the WLC energy at a given wavelength, which was estimated using a procedure previously described [34]. The calculations were carried out using
σ01(λ) extracted from linear absorbance spectra. The higher excited-state lifetime, τS n , was assumed to be one order of magnitude faster (picosecond scale), in accordance with values found in previous studies [31,35]. It is important to note that the results obtained with this fitting procedure are nearly independent of τS n because the population of the Sn state is always very small because of the low fluence of our excitation source. Considering the population redistribution caused by the strong WLC pulse incident on the sample, the time evolution of the nonlinear absorption can be written as:
α (λ , t ) = N [n0 (t )σ 01 (λ ) + n1 (t )σ 12 (λ )]
(5)
Since the detection system in the experiment measures the pulse fluence, the transmittance is calculated by integrating the propagation equation, dI/dz= -α(t)I(t), over the sample thickness and the full pulse duration, which takes into account accumulative effects. From the absorption spectra measured with the WLC Z-scan and using the cross-section σ01(λ), the
σ12(λ) spectra for TMPyP is obtained by fitting of the normalized transmittance acquired at each wavelength. The results obtained will be presented later, together with the spectra of other excited-sates cross-sections. The triplet-triplet absorption measured with the nanosecond laser flash photolysis at 0.5 µs after excitation is depicted in Figure 3. The pump pulse creates triplet states and induces temporal changes in the molecular absorption behavior. Assuming that the molecules that are pumped out from the S0 ground-state can either return to S0 or populate the lowest triplet state T1, via intersystem crossing process, the probe beam will analyze the redistribution of the
8
population between these states. In this way, the transient absorption spectra, ∆A(λ), can be written as [29]: ∆A(λ ) = [ε 34 (λ ) − ε 01 (λ )]CT
(6)
where ε 01 (λ ) and ε 34 (λ ) are the ground and triplet states molar absorption coefficients, respectively, and CT is triplet molar concentration. The relationship between the molar absorption coefficient and absorption cross-section can be obtained from the Beer law expressed as A(λ)= ε(λ)Cℓ, where C is the molar concentration (in mol.L-1). Since the concentration (in molecules/cm3) is given by N = N A3 C , where NA is Avogadro number, Eq. (1) 10
becomes: σ ij (λ ) =
2303 ε ij (λ ) NA
(7)
where the molar absorption coefficient has M-1cm-1 units while the absorption cross-section is in units of cm2. In this way, Eq. (6) can be rewritten by: ∆A(λ ) =
1 [σ 34 (λ ) − σ 01(λ )]NT 2.303
(8)
Since ∆A(λ) was measured with LFP technique, we could, in principle, determine σ34(λ). However, the triplet population (NT) is unknown, which prevents the determination of σ34(λ) from Eq. (8). Many approaches have been introduced to circumvent this problem [29,43,44]. Among them, the saturation method proposed by Kalyanasundaram and Neumann-Spallart for TMPyP porphyrin [43,44] arises as a possible solution. Basically, this method consists in measuring the transient absorbance for different laser intensities up to saturation, which corresponds to the complete transference of molecules to the triplet state. However, in order to avoid the photolysis of the sample, the spectrum is generally obtained in the infrared region, which is a serious limitation because most applications of porphyrins occur in the visible region of the spectrum, particularly in the Q-band region.
9
In our approach, we use PTZ-scan data to determine the absorption cross-section values (σ01 and σ34) at 532 nm for TMPyP in all solvents [35]. These values are then used to calibrate the LFP results by finding NT through Eq. (8). Since the excitation wavelength used in laser flash photolysis experiments is also at 532 nm, we do not measure ∆A(λ) at this wavelength in order to avoid problems with spurious light scattered from the excitation laser beam. Instead, ∆A(λ) for 532 nm is obtained from an extrapolation from their values at 525 and 540 nm. With the value of NT determined from PTZ-scan data, we can use Eq. (8) again in order to obtain the σ34(λ) spectra from the LFP transient absorption spectrum, as depicted in Figure 4 (green full squares). Concerning to the magnitude and spectral profile of the cross-sections, Figure 2 shows that the normalized transmittance is lower than 1 for all solvents investigated which points out to the fact that the molecule presents the absorption cross-section, σ12(λ), higher than that of the ground-state, σ01(λ), in the visible spectral region. The only exception is observed for wavelengths near 518 nm, where the normalized absorbance close to 1 indicates that the absorption cross-sections of both S0→S1 and S1→Sn transitions have similar values. Despite the characteristics of the solvents, the excited singlet state absorption spectra always present a profile quite similar to that assigned to the ground-state absorption. The only difference is the small enhancement observed in the magnitude of the transitions, as shown in Figure 4. Such enhancement of σ12 relative to σ01 is responsible for the increase in the absorption observed in the normalized transmittance spectra. We have studied the singlet excited-state spectra of several organic molecules during the last few years. Our results for indocyanines, phthalocyanines and bisphthalocyanines show that the S0→S1 and S1→Sn cross-sections spectra profiles obtained are completely different [45-47]. On the other hand, we found that for porphyrins and their derivates, like cytochrome and chlorophyll a, the absorption spectra profile are very similar [37,38,42,48,49]. This type of
10
behavior in porphyrin also was previously reported by others authors in others porphyrins [50,51]. In this way, the vibronic characteristic of the Q-band of porphyrins, for both S0→S1 and S1→Sn, are in fact nearly identical with respect to their structuration. These data and the small fluorescence Stokes shift observed previously [35] can be explained by considering that porphyrins are, in general, rigid structures and that the TMPyP vibronic structure is preserved upon excitation. In this way, the molecular structure is the same for all singlet-states participating in the absorption process. Previous work reported in the literature corroborates the suggestion of a strong vibronic coupling between excited-states and ground-state in porphyrins. For instance, Kumble et. al. [52] observed an overall expansion of the macrocycle upon photoexcitation, but the ground-state normal mode structure was retained in the S1 excited state. The vibrational frequencies of both saturated and unsaturated substituents are found to be minimally perturbed in the S1 excited-states. Improta et. al. [53] also observed that the minimum energy geometry of Qx is similar to ground electronic state, owing to the rigid structure of the free-base porphyrin core. Nguyen et al. [54] observed that the lowest triplet excited-state structure of free-base porphyrin and its octahalogenated derivates retain the D2h symmetry. Besides, Sato et. al. [55] suggested the presence of S1-S2 vibronic coupling and the existence of a hyperconjugation between the macrocycle and ethyl groups in the S0, S1 and T1 states. We also note that, similar to S0 → S1 transitions, almost no difference is observed in the excited singlet state absorption spectra of TMPyP for different solvents, strongly suggesting that no appreciable difference in the σ12/σ01 ratio is caused by the changes in the polarity of the medium. Such behavior is better observed in Figure 5(a). Differently from the absorption assigned to singlet excited state, Figure 4 shows that the triplet-triplet absorption (green solid squares) does not have the same vibronic structure present in singlet transitions for most solvents employed. We can see that T1→Tm transitions present
11
vibronic structures at the Q-band region that are completely different from those of singlet states when water, methyl, ethyl, and propyl alcohol are used as solvents. Furthermore, the transition intensities are enhanced in the blue region of the spectrum. By analyzing the solvents properties, we observe in Table 1 that water present a dipole moment higher than the alcohols, which, in principle, points out to a possible influence of the solvent’s dipole moment on the triplet-triplet transition. However, it is also noted in Table 1 that the alcohols used as solvent (ethanol, methanol and propanol), present similar dipole moments and dissimilar Q-band vibronic structuration (see Figures 4(b), 4(c) and 4(d)). Aiming at a deeper understanding of the influence of the solvent on the vibronic structure of the triplet-triplet absorption at the Q-band region, we dissolved the porphyrin in acetonitrile, which is a polar aprotic solvent that presents dipole moment higher than water. In this case, the vibronic structure of the triplet-triplet absorption is more preserved around the Q-band region, when compared to singlet states transitions, than when water is used as solvent. Such result supports that the vibronic structure of triplet states could mainly be affected not only by the solvent dipole moment, but also can be attributed to hydrogen bonds. According to Gensch et al. [56], the nitrogen lone electron pairs in the porphyrin macrocycle are strongly hydrogen-bounded with the water molecules, in the solvation sphere of the porphyrin. This result is in agreement with a larger electron density at the nitrogen atoms of the porphyrin, as found through studies using resonance Raman studies and semiempirical calculations [57,58]. We have recently shown that the photophysical characteristics of TMPyP porphyrin in organic solvents differ from those in water [35], where the formation of hydrogen bonds between TMPyP nitrogen atoms and water hydrogen can explain results such as spectral changes, blue shift and can also explain the higher T1 →Tm absorption cross-section, mainly in Q bands observed in the present work.
12
Previous studies of structural volume changes upon photo-excitation of porphyrins have suggested their contraction upon population of the triplet state [56,59]. It was proposed that this contraction occurs because of the redistribution of the electronic density in the macrocyclic core, which is not observed when the molecule is in the ground-state. Such redistribution could lead to changes in the strength of the hydrogen bonding, between the water molecules in the hydratation shell and the nitrogen atoms in the of porphyrin macrocycle, leading to the molecular contraction of the aqueous sphere around the macrocycle in the triplet state [59]. Finally, Figure 5 presents the ratio spectra σ12/σ01 (a) and σ34/σ01 (b) for all solvents investigated. Such ratios are critical to evaluate if the material investigated has potential to be employed as a good optical limiter [25]. We can see in both spectra that TMPyP is a reverse saturable absorber, presenting higher values of σ12/σ01 for two ranges: from 450 to 500 nm and from 600 to 700 nm. Moreover, above 600 nm, the ground-state cross-section values are very small, implying in reverse saturable absorbers with high linear transmittance. The σ34/σ01 ratio spectrum presents high values between 450 and 500 nm, with an average value close to 15, and reaching the highest value (nearly 26) when the porphyrin is dissolved in methanol. This behavior indicates its potential to optical limiting applications for this spectral region in the nanosecond scale.
IV.
CONCLUSIONS In summary, a new approach to obtain the singlet-singlet and the triplet-triplet excited
states absorption of porphyrins has been presented. In such approach, we employed an association of laser flash photolysis with two extensions of the Z-scan technique to measure both the singlet-singlet and the triplet-triplet excited-state absorption spectra. Employing this method we were able to investigate the influence of different solvents on the ground as well as excited electronic transitions of TMPyP. The results show that S1 → S n spectra have the same
13
vibronic structure as S0 → S1, with similar spectral shape and magnitude, in all solvents employed. However, the T1 → Tm transition shows a quite different vibronic structuration when compared to transitions assigned to singlet states. Finally, our results show that the modification of the T1 → Tm vibronic structure is affected not only by the solvent dipole moment, but also probably by hydrogen bonds established between the porphyrin and solvent molecules.
Acknowledgments - We acknowledge the support from Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq), Fundação de Amparo a Pesquisa do Estado de Minas Gerais (FAPEMIG) and National Institute for Photonics (INCT/INFo). The authors are also in debt with the referees for valuable suggestions.
References [1] D. Koszelewski, A. Nowak-Król, M. Drobizhev, C. J. Wilson, J. E. Haley, T. M. Cooper, J. Romiszewski, E. Górecka, H. L. Anderson, A. Rebane, D. T. Gryko, J. Mater. Chem. C 1 (2013) 2044. [2] E. Güzel, A. Atsay, S. Nalbantoglu, N. Saki, A. L. Dogan, A. Gül, M. B. Koçak, Dyes and Pigments 97 (2013) 238. [3] A. Marcelli, I. J. Badovinac, N. Orlic, P. R. Salvi, Cristina Gellini, Photochem. Photobiol. Sci. 12 (2013) 348. [4] Z. –B. Liu, Z. Guo, X. –L. Zhang, J.-Y. Zheng, J. -G. Tian, Carbon 51 (2013) 419. [5] H. Wang, D. Qi, Z. Xie, W. Cao, K. Wang, H. Shang, J. Jiang, Chem. Commun. 49 (2013) 889.
14
[6] Y. Dwivedi, L. de Boni, P.J. Gonçalves, L.M. Mairink, R. Menegatti, T.L. Fonseca, S.C. Zilio, Spectrochim. Acta A 105 (2013) 483. [7] C. J. P. Monteiro, J. Pina, M. M. Pereira, L. G. Arnaut, Photochem. Photobiol. Sci. 11 (2012) 1223. [8] R. N. Sampaio, W. R. Gomes, D. M. S. Araujo, A. E. H. Machado, R. A. Silva, A. Marletta, I. E. Borissevitch, A. S. Ito, L. R. Dinelli, A. A. Batista, S. C. Zílio, P. J. Gonçalves, N. M. Barbosa Neto, J. Phys. Chem. A 116 (2012) 18. [9] M. Balu, J. Hales, D. J. Hagan, E.W. Van Stryland, Opt. Express 12 (2004) 3820. [10] K. Clays, A. Persoons, Rev. Sci. Instrum. 63 (1992) 3285. [11] L. De Boni, P. L. Franzen, P. J. Gonçalves, I. E. Borissevitch, L. Misoguti, C. R. Mendonça, S. C. Zilio, Opt. Express 19 (2011) 10813. [12] M. Pineiro, A. L. Carvalho, M. M. Pereira, A. M. R. Gonsalves, L. G. Arnaut, S. J. Formosinho, Chem. Eur. J. 4 (1998) 2299. [13] S. M. Abdelghany, D. Schmid, J. Deacon, J. Jaworski, F. Fay, K. M. McLaughlin, J. A. Gormley, J.F. Burrows, D. B. Longley, R. F. Donnelly, C. J. Scott, Biomacromolecules 14 (2013) 302. [14] A. Eichner, F. P. Gonzales, A. Felgenträger, J. Regensburger, T. Holzmann, W. Schneider-Brachert, W. Bäumler, T. Maisch, Photochem. Photobiol. Sci. 12 (2013) 135. [15] J. T. -W. Wang, K. Berg, A. Høgset, S. G. Bown, A. J. MacRobert, Photochem. Photobiol. Sci. 12 (2013) 519. [16] L. M. Almeida, F. F. Zanoelo, K. P. Castro, I. E. Borissevitch, C. M. A. Soares, P. J. Gonçalves, Photochem. Photobiol. 88 (2012) 992. [17] G. B. Rodrigues, F. L. Primo, A. C. Tedesco, G. U. L. Braga, Photochem. Photobiol. 88 (2012) 440. [18] M. Ethirajan, Y. Chen, P. Joshi, R. K. Pandey, Chem. Soc. Rev. 40 (2011) 340.
15
[19] J. M. Dabrowski, L. G. Arnaut, M. M. Pereira, C. J. P. Monteiro, K. Urbańska, S. Simões, G. Stochel, ChemMedChem. 5 (2010) 1770. [20] L. De Boni, D. S. Correa, D. L. Silva, P. J. Gonçalves, S. C. Zilio, G. G. Parra, I. E. Borissevitch, S. Canuto, C. R. Mendonca, J. Chem. Phys. 134 (2011) 014509. [21] F. Gao, X. Wang, S. Wang, M. Liu, X. Liu, X. Ye, H. Li, Tetrahedron 69 (2013) 2720. [22] S. M. O’Flaherty, S. V. Hold, M. J. Cook, T. Torres, Y. Chen, M. Hanack, W. J. Blau, Adv. Mat. 15 (2003) 19. [23] N. M. Barbosa Neto, S. L. Oliveira, L. Misoguti, C. R. Mendonca, P. J. Goncalves, I. E. Borissevitch, L. R. Dinelli, L. L. Romualdo, A. A. Batista, S. C. Zilio, J. Appl. Phys. 99 (2006) 123103. [24] G. de la Torre, P. Vaquez, F. Agullo-Lopez, T. Torres, Chem. Rev. 104 (2004) 3723. [25] C. Loppacher, M. Guggisberg, O. Pfeiffer, E. Meyer, M. Bammerlin, R. Lüthi, R. Schlittler, J. K. Gimzewski, H. Tang, C. Joachim, Phys. Rev. Lett. 90 (2003) 066107-1. [26] D. Voet and J. G. Voet, Biochemistry, John Wiley & Sons Inc. 2o ed., New York, 1995. [27] B. Amand and R. Bensasson, Chem. Phys. Lett. 34 (1975) 44. [28] R. H. Compton, K. T. V. Grattan, T. Morrow, J. Photochem. 14 (1980) 61. [29] I. Carmichael, G. L. Hug, J. Phys. Chem. Ref. Data 15 (1986) 1. [30] H. Gratz, A. Penzkofer, J. Photochem. Photobiol. A 127 (1999) 21. [31] M. Enescu, K. Steenkeste, F. Tfibel, M. P. Fontaine-Aupart, Phys. Chem. Chem. Phys. 4 (2002) 6092. [32] M. Sheik-Bahae, A. A. Said, T. –H. Wei, D. J. Hagan, E. W. Van Stryland, IEEE J. Quantum Electron. 26 (1990) 760. [33] L. Misoguti, C. R. Mendonça, S. C. Zilio, Appl. Phys. Lett. 74 (1999) 1531. [34] L. De Boni, A. A. Andrade, L. Misoguti, C. R. Mendonça, S. C. Zílio, Opt. Express 12 (2004) 3921.
16
[35] P. J. Gonçalves, P. L. Franzen, D. S. Correa, L. M. Almeida, M. Takara, A. S. Ito, S. C. Zílio, I. E. Borissevitch, Spectrochim. Act. A 79 (2011) 1532. [36] D. S. Neto, M. Tabak, J. Colloid Interf. Sci. 381 (2012) 73. [37] R. V. Maximiano, E. Piovesan, S. C. Zílio, A. E. H. Machado, R. de Paula, J. A. S. Cavaleiro, I. E. Borissevitch, A. S. Ito, P. J. Gonçalves, N. M. Barbosa Neto, J. Photochem. Photobiol. A 214 (2010) 115. [38] P. J. Gonçalves, I. E. Borissevitch, S. C. Zilio, Chem. Phys. Lett. 469 (2009) 270. [39] J. Mosinger, M. Janoskova, K. Lang, P. Kubat, J. Photochem. Photobiol. A 181 (2006) 283. [40] P. Kubat, K. Lang, K. Prochazkova, P. Anzenbacher Jr, Langmuir 19 (2003) 422. [41] P. J. Gonçalves, N. M. Barbosa Neto, G. G. Parra, L. de Boni, L. P. F. Aggarwal, J. P. Siqueira, L. Misoguti, I. E. Borissevitch, S. C. Zílio, Opt. Mater. 34 (2012) 741. [42] P. J. Gonçalves, D. S. Corrêa, P. L. Franzen, L. De Boni, C. R. Mendonça, I. E. Borissevitch, S. C. Zílio, Spectrochim. Acta A 12 (2013) 309. [43] K. Kalyanasundaram, Inorg. Chem. 23 (1984) 2453. [44] K. Kalyanasundaram, M. Neumann-Spallart, J. Phys. Chem. 86 (1982) 5163. [45] L. De Boni, E. Piovesan, L. Gaffo, C. R. Mendonça, J. Phys. Chem. A 112 (2008) 6803. [46] M. G. Vivas, E. G. R. Fernandes, M. L. Rodríguez-Méndez, C. R. Mendonca, Chem. Phys. Lett., 531 (2012) 173. [47] L. De Boni, C. R. Mendonca, J. Phys. Org. Chem. 24 (2011) 630. [48] L. De Boni, A.A. Andrade, L. Misoguti, S.C. Zílio, C. R. Mendonça, Opt. Mater. 32 (2010) 526. [49] L. De Boni, D. S. Correa, F. J. Pavinatto, D. S. dos Santos, Jr., C. R. Mendonça, J. Chem. Phys. 126 (2007) 165102. [50] H. Gratz, A. Penzkofer, Chem. Phys. 254 (2000) 363.
17
[51] Z. –B. Liu, Y. Zhu, Y. –Z. Zhu, J. -G. Tian, J. –Y. Zheng, J. Phys. Chem. B 111 (2007) 14136. [52] R. Kumble, G. R. Loppnow, S. Hu, A. Mukherjee, M. A. Thompson, T. G. Spiro, J. Phys. Chem. 99 (1995) 5809. [53] R. Improta, C. Ferrante, R. Bozio, V. Barone, Phys. Chem. Chem. Phys. 11 (2009) 4664. [54] K. A. Nguyen, P. N. Day, R. Pachter, J. Phys. Chem. A 104 (2000) 4748. [55] S. Sato, K. Aoyagi, T. Haya, T. Kitagawa, J. Phys. Chem. 99 (1995) 7766. [56] T. Gensch, C. Viappiani, S.E. Braslavsky, J. Am. Chem. Soc. 121 (1999) 10573. [57] S. E. J. Bell, C. B. Aakeroy, A. H. R. Al-Obaidi, J. N. M. Hegarty, J. J. McGarvey, C. R. Lefley, J. N. Moore, R. E. Hester, J. Chem. Soc., Faraday Trans. 91 (1995) 411. [58] S. E. J. Bell, A. H. R. Al-Obaidi, J. N. M. Hegarty, R. E. Hester, J. J. McGarvey, J. Phys. Chem. 97 (1993) 11599 [59] M. M. Kruk, S. E. Braslasky, Photochem. Photobiol. Sci. 11 (2012) 972.
18
Figure Captions
Figure 1. Five-energy-level diagram.
Figure 2. Normalized transmittance spectra for TMPyP in the presence of solvents with different polarities.
Figure 3. Absorption spectra obtained with LFP at 0.5 µs after excitation for TMPyP in the presence of solvents with different polarities.
Figure 4. Absorption cross-section spectra of ground (σ01, inverted full triangles), first excited singlet (σ12, open red circles) and first excited triplet states (σ34, full green squares) obtained for TMPyP in different polar protic solvents: (a) water, (b) ethyl, (c) methyl, (d) propyl alcohol and (e) acetonitrile.
Figure 5. (a) σ12/σ01 and (b) σ34/σ01 spectra for all solvents investigated.
19
Figure 1. N. M. Barbosa Neto et. al.
Normalized Transmittance
1.00
0.95 H2 0 0.90
Acetonitrile MeOH EtOH PrOH
0.85
0.80 450
500
550
600
Wavelength (nm) Figure 2. N. M. Barbosa Neto et. al.
650
700
A
0.16
H2 O
0.12
Acetonitrile MeOH EtOH PrOH
0.08
0.04
0.00 400
500
600
Wavelength (nm) Figure 3. N. M. Barbosa Neto et. al.
700
2
cm )
10
12()
6
34()
4 2 0 500
550
600
650
700
Wavelength (nm)
5 0
40
8
01()
2 -17
ij - cross-section (10
35
cm )
(b)
30 25 20
6
12() 34()
4
2
0 500
15
550
600
650
700
Wavelength (nm)
10 5 0 15
8
01()
ij - cross-section(10
-17
2
cm )
(c) 10
6
12() 34()
4
2
0 500
5
0 25
-17
2
cm )
(d)
ij - cross-section (10
20 15 10
550
600
Wavelength (nm)
650
8
700
01()
6
12()
4
34()
2
0 500
550
600
650
700
Wavelength (nm)
5 0 20
8
01( )
2
cm )
2
(e)
-17
-17
ij - cross-section (10
01()
8
-17
15
cm )
-17
ij - cross-section (10
10
ij - cross-section (10
20
6
ij - cross-section(10
2
cm ) ij - cross-section (10-17 cm2)
-17
(a)
2
cm ) - cross-section (10-17 cm2) ij - cross-section (10 ij
25
15
10
12( ) 34( )
4
2
0 500
550
600
Wavelength (nm)
650
700
5
0 450
500
550
600
650
Wavelength (nm)
Figure 4. N. M. Barbosa Neto et. al.
700
21/01
2.5
(a)
H2O MeOH EtOH PrOH Acet.
2.0
1.5
1.0
30
(b)
H2O MeOH EtOH PrOH Acet.
25
34/01
20 15 10 5 0 450
500
550
600
650
Wavelength (nm) Figure 5. N. M. Barbosa Neto et. al.
700
Table 1.
Solvent Dipole moment (D) Type Water 1.85 Polar protic Ethanol 1.69 Polar protic Methanol 1.70 Polar protic Propanol 1.68 Polar protic Acetonitrile 3.92 Polar aprotic
20
Highlights We have obtained the excited states spectra of triplet and singlet states. It was employed UV/Vis absorption, flash photolysis and Z–scan techniques. S1 → Sn absorption spectra were obtained by white-light Z-scan technique. T1 → Tm absorption was obtained by flash photolysis and pulse train Z-scan. The vibronic structuration observed is probably caused by hydrogen bonds.
21
22